Thank you for the inputs Mmathis and JamieB.
To give some more background: I don't have fish. The pond is at least 10 years old. I live in Oregon, where it rains a lot.
Yes I understand that algae might be helping the ecosystem. We never had a pond before. Our only experience with water bodies in the backyard was from a swimming pool, for which looking green means it's not well maintained (which happened to the pool at a friend's house
). So we'd prefer to have clear water in the pond - or at least to have less algae. And yes my follow-up question would be how to keep the pond algae-free (I was thinking something like adding chemicals - well it doesn't have to be chemicals - to keep the pH at a certain level, but maybe it should be to control the nutrient level).
So sounds like adding plants would help. Speaking about nutrients, for a pond that uses circulated water, what are the sources of nutrients? Leaves and